Optical communications systems typically require optical receivers capable of rapidly delivering large voltage or current responses to incident optical signals.
One approach for obtaining a high speed optical receiver is to use a low-temperature grown gallium arsenide MSM (metal-semiconductor-metal) photoconductor. For these devices, source and drain contacts are made directly to the low-temperature grown gallium arsenide, and an incident optical signal is used to increase the channel conductance. In operation, illumination causes the photoconductor to absorb photons, which decreases the resistivity of the device. For low-temperature grown gallium arsenide, photoinduced conductance increases by a factor of 10.sup.6 have been observed. Low-temperature grown photoconductors exhibit fast turn-on response times because the photogenerated carriers have a short lifetime, typically less than 20 picoseconds. A disadvantage to the low-temperature grown photoconductor, however, is that it exhibits a long turn-off transient, typically greater than 10 nanoseconds.
Another approach uses an optically switched resonant tunneling device. Certain resonant tunneling devices exhibit opto-electric characteristics. These devices switch between a high conductance and low conductance electrical state depending on the level of illumination thereon. In the absence of light, the detector quickly returns to its original high conductance operating state, unlike the photoconductor, which is plagued by a long turn-off transient. A disadvantage of the optically switched resonant tunneling devices, however, is that the response times are slow compared to low-temperature grown photoconductors. A further disadvantage is that it is difficult to optimize the resonant tunneling device structure for direct optical interaction since the tunneling distance is much less than the absorption length.
Still another approach uses a PIN photodiode integrated with a resonant tunneling device. This approach uses a PIN photodiode as an optical detector, and the resonant tunneling device as a switch. This combination operates in a photo-current mode, which creates a disadvantage of having to carefully match the PIN photocurrent to the peak current of the resonant tunneling device. A further disadvantage of this approach is that the combination requires a high intensity incident optical signal. Still another disadvantage of this combination is that it has a slower response time than the low-temperature grown photoconductor.